After mashing, sweet wort is boiled. This stops the degradative proc-
esses by inactivating the malt enzymes. It also pasteurizes the wort,
completes ionic interactions such as calcium phosphate precipitation,
denatures and precipitates proteins and tannins which separate as a
material known as hot break or trub and helps dissolve any sugars which
may be added at this stage as an adjunct.
Hops are also added during boiling. These are the cones or strobili of
the plantHumulus lupuluswhose principal purpose is the bittering of the
wort. The hop resin containsa-acids such as humulone and cohumulone
which are only partially soluble in wort. During boiling they isomerize to
isohumulones which are more soluble and more bitter than a-acids
(Figure 9.12). Although hop resins have some antibacterial action, they
play little part in assuring the bacteriological stability of beer as spoilage
bacteria such as lactobacilli rapidly acquire a tolerance to them.
Wort boiling lasts for 1–2 h during which 5–15% of the volume is
evaporated. Hop residues are then strained off, hot trub is removed in a
whirlpool separator, and the hopped wort cooled to the fermentation
temperature.
The yeasts used to brew ales and lagers are strains ofSaccharomyces
cerevisiae, known asS. cerevisiaevar.cerevisiaeandS. cerevisiaevar.
carlsbergensis (uvarum)respectively. The distinctions between the yeasts
used in ale and lager brewing are slight. Traditionally, ale yeasts were
regarded as top fermenters which formed a frothy yeast head on the
surface of brewing beer and was skimmed off to provide yeasts for
pitching (inoculating) subsequent batches, while lager yeasts were bot-
tom fermenters which formed little surface head and were recovered
from the bottom of the fermenter. Nowadays this is a less useful
distinction as many ales are brewed by bottom fermentation.
The cardinal temperatures of the two organisms differ and this is
reflected in the different temperatures used for lager fermentations (8–
121 C) and for ale fermentations (12–18 1 C). They can also be distin-
guished by the ability ofS. cerevisiaevar.carlsbergensisto ferment the
disaccharide melibiose, although this is of no practical import since the
sugar does not occur in wort.
During fermentation the yeast converts fermentable carbohydrate to
ethanol via the EMP pathway. Although this is an anaerobic process, a
vigorous fermentation is often helped by aeration of the wort before
pitching with yeast. This supplies oxygen, necessary for the synthesis of
unsaturated fatty acid and sterol components of the yeast cell membrane,
and may sometimes be repeated later in the fermentation.
A time course of a typical ale fermentation is shown in Figure 9.13. After
an initial vigorous phase during which there is active yeast growth, ethanol
production and a drop in pH as nitrogen is removed from the wort, there is
a second phase of slower ethanol production in the absence of further yeast
352 Fermented and Microbial Foods